Ethylbenzene is one of the aromatic hydrocarbons that is obtained from naphtha pyrolysis or in reformate. Reformate is an aromatic product given by the catalysed conversion of straight-run hydrocarbons boiling in the 70 to 190° C. range, such as straight-run naphtha. Such hydrocarbons are themselves obtained by fractionation or distillation of crude petroleum oil, their composition varying depending on the source of the crude oil, but generally having a low aromatics content. On conversion to reformate, the aromatics content is considerably increased and the resulting hydrocarbon mixture becomes highly desirable as a source of valuable chemicals intermediates and as a component for gasoline. The principle components are a group of aromatics often referred to as BTX: benzene, toluene, and the xylenes, including ethylbenzene. Other components may be present such as their hydrogenated homologues, e.g. cyclohexane.
Of the BTX group the most valuable components are benzene and the xylenes, and therefore BTX is often subjected to processing to increase the proportion of those two aromatics: hydrodealkylation of toluene to benzene and toluene disproportionation to benzene and xylenes. Within the xylenes, para-xylene is the most useful commodity and xylene isomerisation or transalkylation processes have been developed to increase the proportion of para-xylene.
A further process that the gasoline producer can utilize is the hydrodealkylation of ethylbenzene to benzene.
Generally, the gasoline producer will isolate BTX from the reformate stream, and then subject the BTX stream to xylene isomerisation with the aim of maximising the para-xylene component. Xylene isomerisation is a catalytic process; some catalysts used in this process have the ability not just to isomerise xylenes but also simultaneously to dealkylate the ethylbenzene component. Normally the para-xylene is then separated out to leave benzene, toluene (unless toluene conversion processes have already been applied) and the remaining mixed xylenes, including ethylbenzene. This BTX stream can either be converted by transalkylation to increase the yield of xylenes by contacting with a heavier hydrocarbon stream or can be converted by dealkylation to eliminate selectively ethylbenzene and to increase the yield of benzene, while allowing the xylenes to reach equilibrium concentrations. The latter process is the subject of the present invention.
In ethylbenzene dealkylation at this latter stage of BTX treatment, it can be desirable to have a catalyst with high activity. Such catalyst makes it possible to operate at high weight hourly space velocities. Additionally, it is advantageous if a catalyst has high flat plate crushing strength as this leads to less fines and broken material being formed during handling and catalyst loading or unloading from the reactor. Fines are known to cause problems in operation such as contributing significantly to the pressure drop over a reactor.
Ethylbenzene dealkylation catalysts are well known in the art and typically comprise platinum on a zeolite containing support as described for example in EP-A-0018498.
WO-A-2009/016143 relates to ethylbenzene dealkylation catalysts comprising pentasil zeolite having a bulk silica to alumina ratio in the range of from 20 to 150, platinum and tin.
U.S. Pat. No. 4,582,815 describes a method for preparing silica-rich solids which comprises mixing silica-rich solids with water in an alkali metal base or basic salt followed by mulling, extruding, drying and neutralizing the base before calcination. Calcining can cause the alkali metal to become trapped, perhaps by encapsulation, and it is then difficult to remove by ion exchange and is usually removed incompletely. The products are described to be suitable for a wide variety of processes which are both non-catalytic and catalytic such as hydrocracking, isomerization, hydrogenation, dehydrogenation, polymerization, reforming, catalytic cracking and catalytic hydrocracking.